An electrically conductive paste made by dispersing a silver powder in a thermosetting resin composition is cured by heating to form an electrically conductive film, therefore it is used for the formation of an electrically conductive circuit on a printed circuit substrate; the formation of the electrodes of various display elements and various electronic components, such as a resistor and a capacitor; the formation of an electrically conductive film for an electromagnetic wave shield; the adhesion of a chip component, such as a capacitor, a resistor, a diode, a memory, and an arithmetic element (CPU), to a substrate; the formation of the electrodes of a solar cell, particularly, the electrodes of a solar cell using an amorphous silicon semiconductor that cannot be treated at high temperature; the formation of the external electrode of a chip-type ceramic electronic component, such as a laminated ceramic capacitor, a laminated ceramic inductor, and a laminated ceramic actuator; and the like.
Recently, because of high performance of chip components, the amount of heat generation from the chip components increases, therefore, an improvement in thermal conductivity as well as electrical conductivity has been required. Therefore, it is considered that electrical conductivity and thermal conductivity are improved by increasing the content of silver particles as much as possible, but there is the problem that the viscosity of the electrically conductive paste increases, significantly decreasing workability.
On the other hand, a highly-dispersible spherical silver powder optimal for producing an electrically conductive paste that can form the electrodes and circuits of chip components, plasma display panels, and the like, with high density, high precision, and high reliability; and a silver paste using the same are taught in Patent Document 1. Also, as a method for applying an electrically conductive paste to a circuit board, an ink jet method, in addition to a screen printing method, has begun to be used. Therefore, a silver ink comprising a silver powder in which the average particle diameter DIA of primary particles obtained by the image analysis of a scanning electron microscope image is 0.6 μm or less, polyols, and further a viscosity-adjusting agent and the like as required is proposed (see Patent Document 2).
In Patent Document 1, a silver powder is prepared by mixing and reacting an aqueous solution of silver nitrate and aqueous ammonia to prepare an aqueous solution of a silver ammine complex, contacting and reacting this aqueous solution of a silver ammine complex and an aqueous solution of hydroquinone, anhydrous potassium or ammonium sulfite, and gelatin to precipitate a silver powder by reduction, filtering this reaction liquid, washing the residue with water, and drying it by heating. Patent Document 1 describes that thus prepared silver powder was used to prepare a silver paste, but its formulation is unclear. Then, when the present inventors mixed the silver powder with a small amount of water to make them pasty and heated, it was found that there were the problem that the silver powder was not sintered sufficiently and the problem that even if it was sintered to produce solid silver, strength, electrical conductivity, and thermal conductivity were unexpectedly low. In Patent Document 2, a silver powder is prepared by mixing and reacting an aqueous solution of silver nitrate and aqueous ammonia to prepare an aqueous solution of a silver ammine complex, contacting and reacting this aqueous solution of a silver ammine complex and an organic reducing agent (such as hydroquinone, ascorbic acid, and glucose), particularly an aqueous solution of hydroquinone, to precipitate a silver powder by reduction, filtering this reaction liquid, washing the residue with water and methanol, and drying it by heating. However, when the present inventors heated a silver ink comprising thus prepared silver powder and polyols (for example, 1,4-butanediol, dipropylene glycol), it was found that there were the problem that the silver powder was not sintered sufficiently and the problem that if it was sintered to produce solid silver, strength, electrical conductivity, and thermal conductivity were unexpectedly low.
Patent Document 1: Japanese Patent Laid-Open No. 2001-107101, i.e., JP 2001-107101 A
Patent Document 2: Japanese Patent Laid-Open No. 2005-93380, i.e., JP 2005-93380 A
As a result of studying diligently to develop a silver paste, that is, a pasty silver particle composition without the above problems, the present inventors have found that the shape, particle diameter, and carbon content in silver particles and a coating agent for the silver surface affect the sintering property of the silver particles, and the strength, electrical conductivity, and thermal conductivity of solid silver produced by sintering, to complete the present invention. An object of the present invention is to provide a pasty silver particle composition in which silver particles are easily sintered by heating to provide solid silver having excellent strength, electrical conductivity, and thermal conductivity; a process for producing solid silver having excellent strength, electrical conductivity, and thermal conductivity from the pasty silver particle composition; solid silver having excellent strength, electrical conductivity, and thermal conductivity; a method for joining metal members strongly with good electrical conductivity and thermal conductivity, using the pasty silver particle composition; and a process for producing a printed wiring board having silver wiring having excellent wear resistance, adhesive property to the substrate, electrical conductivity, and thermal conductivity.